A star found on the red part of the → horizontal branch.
According to theoretical models, these stars result from the evolution of stars with a
mass around 0.8 Msun, higher than that giving rise to
→ BHB stars. Upon helium burning in their cores,
the remnant envelope of the red giant collapses.

A star that has a non-zero → angular velocity.
In a rotating star, the → centrifugal forces reduce the
→ effective gravity according to the latitude and also
introduce deviations from sphericity. In a rotating star, the equations of stellar
structure need to be modified. The usual spherical coordinates must be replaced
by new coordinates characterizing the → equipotentials.
See also → von Zeipel theorem.

A member of a large class of yellow pulsating stars of type A2-F6 with
periods less than 1 day. They are similar to Cepheids, except that
their periods are much shorter and are less luminous. RR Lyrae stars
belong to Population II and are often found in globular clusters
(hence one of their older names - cluster variables) or elsewhere in
the galactic halo. They are used as distance indicators out to more
than 200 kpc.

A massive, young, and hot star that is moving quickly
through space. Runaways are probably propelled through space
from a binary star when its companion has exploded as a supernova, or ejected from
a stellar cluster by the dynamical interactions in the system.

A general whole-sky catalog compiled by the Smithsonian Astrophysical Observatory
which results from the combination of several earlier catalogs.
The compilation gives positions and proper motions for 258,997 stars,
of which 8,712 are double and 499 variable, with an average
distribution of 6 stars per square degree. The star positions have an
average standard deviation of 0''.2 at their original epochs (0''.5 at
epoch 1963.5). The equinox is 1950.0 and the system that of the FK4.

The formation of second-generation stars in a → molecular cloud,
as triggered by the presence of → massive stars.
The observation that some nearby → OB associations
contain distinct, spatially separate subgroups of → OB stars
in a sequence of monotonically changing age led Blaauw (1964, ARA&A 2, 213)
to suggest that star formation in fact occurs in sequential bursts during
the lifetimes of the corresponding molecular clouds. The first quantitative model
of this mechanism was presented by Elmegreen and Lada (1977, ApJ 214, 725), who
showed that the powerful ultraviolet photons of the massive star create an
→ ionization front which advances in the molecular cloud
and is preceded by a → shock front. The compressed
neutral gas lying between the ionization and shock fronts
is gravitationally unstable and collapses in time-scales of a few million years
to form a new generation of massive stars. The propagation of successive births
of OB groups would produce a chain of associations presenting a gradient of age.
Elmegreen and Lada estimated the propagation velocity
to be 5 km s-1. For a region with a length larger than 100 pc,
this would imply an age difference of the order of 20 million years between the
extremities. See also → stimulated star formation,
→ triggered star formation;
→ collect and collapse model.

A main-sequence star, usually of spectral class B to F, whose
spectrum shows bright emission lines superimposed on the normal absorption lines.
The emission spectrum is explained by the presence of a
circumstellar shell of gas surrounding the star at the equator.
Shell stars are fast rotators.

A member of a class of → B stars that
are situated along the → main sequence with
→ spectral types ranging from B2 to B9 and masses from 3 to 7
→ solar masses. In the → H-R diagram
the SPB group lies below → beta Cephei variables,
which are more massive. SPBs show light and line-profile variations that are
multi-periodic with periods of the order of days. This variability is
understood in terms of non-radial → stellar pulsations, and their
→ oscillation modes are high-order
→ g modes. Theoretical models attribute
the pulsational nature of SPBs to the → kappa mechanism,
acting in the metal → opacity bump at 2 x 105 K.
Their g-mode pulsations penetrate deep into the stellar interior, making these
objects very promising for → asteroseismology. Several
oscillation modes are excited simultaneously, resulting in periodicities on time scales
of the order of months or even years. The prototype of this group is 53 Per.
First introduced as a distinct class by Waelkens (1991, A&A 246, 453).